/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode Merge(ListNode list1,ListNode list2) {
        if(list1==null &&list2==null) return null;
        ListNode head=new ListNode(-1);//关键
        ListNode temp=head;
        while(list1!=null && list2!=null){
            if(list1.val<list2.val){
                temp.next=list1;
                list1=list1.next;
            }else {
                temp.next=list2;
                list2=list2.next;
            }
            temp=temp.next; //这个不能忘
        }
        if(list1!=null){
            temp.next=list1;
        }
        if(list2!=null){
            temp.next=list2;
        }
        return head.next;//关键
    }
    
}